The present invention relates to a surface acoustic wave (SAW) device for use in radio frequency circuits and the like of a wireless communications device.
In an application where the pass band is relatively wide and the flatness of the phase characteristics within the pass band is important as in filters for use in the IF stage in CDMA devices which are drawing attention in recent years, a surface acoustic wave device is suitable as the amplitude characteristics and the phase characteristics can be independently designed. Also, with the progress of downsizing and weight reduction of mobile terminals, miniaturization of surface acoustic wave devices for IF stage is also being demanded. Accordingly, improved versions of transversal type surface acoustic wave devices are being developed.
In a conventional surface acoustic wave device, as shown in FIG. 14, input and output interdigital transducer electrodes (hereinafter IDT electrodes) 101 and 102 are provided in parallel on a piezoelectric substrate 100, first reflector electrodes 103 are provided between the end on the output side of the input IDT electrode and an end of the piezoelectric substrate 100, and second reflector electrodes 104 are provided between the end on the input side of the output IDT electrode 102 and an end of the piezoelectric substrate 100. Also, the first and second reflector electrodes 103, 104 are obliquely provided relative to the direction perpendicular to the direction of propagation of surface acoustic waves.
When an electrical signal is inputted to the input terminal of this surface acoustic wave device, surface acoustic waves propagate through the input IDT electrode 101, reflect at the first reflector electrodes 103, propagate to the second reflector electrodes 104, reflect at the second reflector electrodes 104, propagate to the output IDT electrode 102, and are put out from the output terminal.
Accordingly, when compared with a transversal type surface acoustic wave device, the size can be made smaller by the amount of overlap of the input and output IDT electrodes 101, 102 in the direction of propagation of surface acoustic waves.
In a surface acoustic wave device having this structure, the state of reflection of surface acoustic waves differs depending on the angle of inclination of the first and second reflector electrodes 103, 104 relative to the input and output IDT electrodes 101, 102, thus affecting the in-passband characteristics.
It is therefore an object of the present invention to provide a surface acoustic wave device that can efficiently transmit surface acoustic waves from the input IDT electrode to the output IDT electrode and hence has a superior in-passband characteristic.
In order to achieve the above object, a first surface acoustic wave device of the present invention comprises a piezoelectric substrate, input and output IDT electrodes provided on the piezoelectric substrate in a manner such that the directions of propagation of surface acoustic waves are in parallel, a first reflector electrode disposed on the side of output of the surface acoustic waves of the input IDT electrode, and a second reflector electrode provided on the side of input of the surface acoustic waves of the output IDT electrode, and satisfies Equation (1). As it can efficiently propagate surface acoustic waves, it provides a superior in-passband characteristic.
2(xcex8xe2x88x925)xe2x89xa6tanxe2x88x921(D/L)xe2x89xa62(xcex8+5)xe2x80x83xe2x80x83(1)
where
xcex8: angle of inclination (in degrees excluding zero degree) of the first or the second reflector electrode relative to a plane perpendicular to the direction of propagation of surface acoustic waves of the input and output IDT electrodes;
D: center-to-center distance in xcexcm of the widths of the input and output IDT electrodes in the direction perpendicular to the direction of propagation of surface acoustic waves:
L: center-to-center distance in xcexcm of the first reflector electrode and the second reflector electrode.
In addition, by designing the value of xcex8 to be equal to or smaller than 25xc2x0, this device may be made further superior in its in-passband characteristics.
Furthermore, by designing the reflection coefficients of the first and second reflector electrodes to be approximately equal to unity, insertion loss of the device may be lowered.
Yet furthermore, by designing at least one of the input and output IDT electrodes to be a unidirectional electrode, insertion loss of the device may be lowered.
Yet furthermore, by electrically connecting as well as grounding the first and second reflector electrodes of the device with a connect electrode provided between the input and output IDT electrodes, the cable run electrode works as a shielding electrode and prevents electromagnetic coupling between the input and output IDT electrodes thereby increasing the quantity of out-of-passband attenuation.
Yet furthermore, by dividing the first and second reflector electrodes of the device into more than one unit, the quantity of out-of-passband attenuation may be further increased.
Yet furthermore, by making the cable run electrode broader than the bus bars of the input and output IDT electrodes of the device, shielding effect may be further enhanced.
Yet furthermore, with this device, by providing on at least one of the ends on the sides of the input and output IDT electrodes of the first or the second reflector electrodes an electrode finger having the same pitch, width, and angle of inclination as the electrode fingers of the above-mentioned reflector electrodes but shorter in length than the above-mentioned electrode fingers, a filter waveform close to a rectangle may be realized.
Yet furthermore, with this device, by providing at the end on the IDT electrode side of the reflector electrodes provided with a short electrode finger and in parallel with the electrode fingers of the IDT electrodes an electrode finger, having a width approximately equal to xe2x85x9 of the wavelength of surface acoustic waves, for connecting ends of electrode fingers, unwanted reflection may be prevented.
Yet furthermore, with this device, by making the reflection characteristics of the first reflector electrodes and the second reflector electrodes different, the quantity of out-of-passband attenuation may be increased.
Yet furthermore, with this device, by making the metalization ratios of the first and second reflector electrodes to be in the range 0.45 to 0.75, reflection efficiency per electrode finger of the reflector electrodes may be enhanced thus enabling size reduction of the reflector electrodes.
Yet furthermore, with this device, the quantity of out-of-passband attenuation may be increased by making the width of some of the electrode fingers of the reflector electrodes greater than the width of other electrode fingers.
Yet furthermore, with this device, by providing a third reflector electrode on the side opposite the side where the first reflector electrode of the input IDT electrode is provided at a predetermined distance from the input IDT electrode, and providing a fourth reflector electrode on the side opposite the side where the second reflector electrode of the output IDT electrode is provided at a predetermined distance from the output IDT electrode, bi-directional surface acoustic waves propagating through the input IDT electrode may be efficiently propagated to the output IDT electrode.
Yet furthermore, with this device, by configuring at least one of the input and output IDT electrodes with a split electrode having an electrode finger width approximately equal to xcex/8 (xcex: wavelength of surface acoustic waves), internal reflection within the input and output IDT electrodes may be prevented and a wide and flat pass band may be provided.
Yet furthermore, with this device, when the input and output terminals of the surface acoustic wave device are of unbalanced type, by grounding the input and output terminals on the side of opposing input and output IDT electrodes, electromagnetic coupling between the input and output IDT electrodes may be prevented and a large quantity of out-of-passband attenuation may be obtained.
Yet furthermore, with this device, by providing an acoustic absorber between the first and second reflector electrodes and the end in the direction of propagation of surface acoustic waves of the piezoelectric substrate, unwanted reflected waves at the end of the piezoelectric substrate may be absorbed and a flat pass band may be obtained.
Yet furthermore, with this device, by providing a sound-absorbing material also between the first and second reflector electrodes and the end parallel to the direction of propagation of surface acoustic waves of the piezoelectric substrate, unwanted reflected waves at the end of the piezoelectric substrate may be further absorbed and a flat pass band may be obtained.
A second surface acoustic wave device of the present invention comprises a piezoelectric substrate, input and output IDT electrodes provided in parallel on the piezoelectric substrate, first to fourth reflector electrodes provided on both sides of the input and output IDT electrodes, and a fifth reflector electrode provided between the input and output IDT electrodes, where the first to the fourth reflector electrodes are inclined by roughly the same angles relative to a plane perpendicular to the direction of propagation of surface acoustic waves, thus providing a surface acoustic wave device small in size and superior in the quantity of out-of-passband attenuation.
Additionally, with this device, by satisfying Equation (2), surface acoustic waves may be efficiently propagated.
2(xcex8xe2x88x925)xe2x89xa6tanxe2x88x921(D/L)xe2x89xa62(xcex8+5)xe2x80x83xe2x80x83(2)
where
xcex8: angle of inclination (in degrees excluding zero degree) of one or more of the first to the fourth reflector electrodes relative to a plane perpendicular to the direction of propagation of surface acoustic waves of the input and output IDT electrodes;
D: center-to-center distance in xcexcm of the widths of the input and output IDT electrodes in the direction perpendicular to the direction of propagation of surface acoustic waves;
L: center-to-center distance in xcexcm of the first reflector electrode and the second reflector electrode.
In addition, by designing the value of xcex8 to be equal to or smaller than 25xc2x0, this device may be made further superior in its in-passband characteristics.
Furthermore, by designing the reflection coefficients of the first to fifth reflector electrodes to be approximately equal to unity, insertion loss of the device may be lowered.
Yet furthermore, with this device, by configuring at least one of the input and output IDT electrodes with a split electrode having an electrode finger width approximately equal to xcex/8 (xcex: wavelength of the surface acoustic wave), internal reflection inside the input and output IDT electrodes may be prevented and a wide and flat pass band may be provided.
Yet furthermore, with this device, by making the fifth reflector electrode work as a shielding electrode by grounding it, electromagnetic coupling between the input and output IDT electrodes may be prevented thus providing a device having a large quantity out-of-passband attenuation.
Yet furthermore, with this device, by providing on at least one of the ends on the side of the input and output IDT electrodes of the first to the fourth reflector electrodes an electrode finger having the same pitch, width, and angle of inclination as the electrode fingers of the reflector electrodes but shorter in length than the electrode fingers, a filter waveform close to a rectangle may be obtained.
Yet furthermore, with this device, by providing at the end on the IDT electrodes side of the reflector electrode provided with a short electrode finger and in parallel with the electrode fingers of the IDT electrodes an electrode finger having a width approximately equal to xe2x85x9 of the wavelength of surface acoustic waves for connecting ends of electrode fingers, unwanted reflection may be prevented.
Yet furthermore, with this device, by making the reflection characteristics of the first and second reflector electrodes different from those of the third and fourth reflector electrodes, the quantity of out-of-passband attenuation may be increased.
Yet furthermore, with this device, by making the metalization ratios of the first to the fifth reflector electrodes to be in the range 0.45 to 0.75, reflection efficiency per electrode finger may be enhanced thus enabling size reduction of the reflector electrodes.
Yet furthermore, with this device, the quantity of out-of-passband attenuation may be increased by making the width of some of the electrode fingers of at least one of the first to the fifth reflector electrodes greater than the width of other electrode fingers.
Yet furthermore, with this device, when the input and output terminals of the surface acoustic wave device are of unbalanced type, by grounding the input and output terminals on the side of opposing input and output IDT electrodes, electromagnetic coupling between the input and output IDT electrodes may be prevented and a large quantity of out-of-passband attenuation may be obtained.
Yet furthermore, with this device, by providing a sound-absorbing material between the first to the firth reflector electrodes and the end of the piezoelectric substrate in the direction of propagation of surface acoustic waves, unwanted reflected waves at the end of the piezoelectric substrate may be absorbed and a flat pass band may be obtained.
Yet furthermore, with this device, by providing a sound-absorbing material also between the first to the fourth reflector electrodes and the end of the piezoelectric substrate in the direction parallel to the direction of propagation of surface acoustic waves, unwanted reflected waves at the end of the piezoelectric substrate may be further absorbed and a flat pass band may be obtained.
A communications device in accordance with the present invention is one including a mixer, a surface acoustic wave device in accordance with the present invention of which the input side is connected to the output side of the mixer, and an amplifier of which the input side is connected to the output side of the surface acoustic wave device, and enabling reduction in the number of factors related to amplifier elements, or reduction in the amplifier power dissipation thus providing superior cost performance.